Meanwhile, cook pancetta in heavy large skillet over medium-high heat until golden brown, about 3 minutes. Using slotted spoon, transfer pancetta to paper towels. Add mushrooms and 2 tablespoons oil to drippings in skillet. Sauté until mushrooms are tender, about 6 minutes. Stir in kale and pancetta. Add garlic and broth; simmer until broth is slightly reduced, about 6 minutes. Stir in thyme, lemon peel, and 2 tablespoons oil. Season to taste with salt and pepper.

Whisk butter and Parmesan into polenta and divide among plates. Top with kale mixture.